**3.4 Designs of digester**

The most common digester design is cylindrical. Digesters can be classified in horizontal and vertical designs (Fig. 3). Currently, vertical concrete or steel digesters with rotating propellers or immersion pumps for homogenization are widespread. Vertical tanks simply take feedstock in a pipe on one side, whilst digestate overflows through a pipe on the other side. In horizontal plug-flow systems, a more solid feedstock is used as a plug that flows through a horizontal digester at the rate it is fed-in. Vertical tanks are simpler and cheaper to operate, but the feedstock may not reside in the digester for the optimum period of time. Horizontal tanks are more expensive to build and operate, but the feedstock will neither leave the digester too early nor stay inside the digester for an uneconomically long period.

Fig. 3. Horizontal (a) and vertical (b) digester (Gronauer and Neser, 2003)

The most common digester design is cylindrical. Digesters can be classified in horizontal and vertical designs (Fig. 3). Currently, vertical concrete or steel digesters with rotating propellers or immersion pumps for homogenization are widespread. Vertical tanks simply take feedstock in a pipe on one side, whilst digestate overflows through a pipe on the other side. In horizontal plug-flow systems, a more solid feedstock is used as a plug that flows through a horizontal digester at the rate it is fed-in. Vertical tanks are simpler and cheaper to operate, but the feedstock may not reside in the digester for the optimum period of time. Horizontal tanks are more expensive to build and operate, but the feedstock will neither leave the digester too early nor stay inside the digester for an uneconomically long period.

(a)

(b)

Fig. 3. Horizontal (a) and vertical (b) digester (Gronauer and Neser, 2003)

**3.4 Designs of digester** 
